Gas generators, particularly those having application as emergency power units for military aircraft, are small, lightweight, intermittent duty power supply devices which ensure instant, reliable power output even after relatively long dormant periods. Such gas generators typically utilize a pulsed spray of liquid hydrazine into a catalyst bed such as Shell 405, in which production of hot gas occurs. The gas generated by the device may then be used to drive a turbine wheel, which may power such devices as engine starters, electrical generators, and hydraulic pumps.
Gas generators for military applications are designed to be reusable, with only a minimal amount of maintenance being required between uses. It is therefore imperative that the gas generator be capable of a consistent and reliable level of performance involving repeated usage over an extended period of time without requiring major overhaul, rebuilding, or replacement of the catalyst.
For optimum performance, the catalyst in the gas generator is located in two distinct beds, a primary bed into which the liquid hydrazine is introduced by a spray nozzle device, and a secondary bed interposed between the primary bed and the location in the generator at which the gas generated exits the device. The liquid hydrazine initially decomposes into a gas in the first catalyst bed, and the characteristics of the gas exiting the device are shaped as the gas travels through the second catalyst bed, reaching a high temperature.
Liquid hydrazine is supplied to the gas generator at a pressure of approximately 400 p.s.i., and gas leaving the generator is at approximately the same pressure. The gas generator is therefore controlled by the amount of liquid hydrazine sprayed into the catalyst beds, which amount determines the volume of gas produced by the gas generator, and therefore controls the speed at which a turbine driven by the gas generator will operate.
In order to precisely control the operating speed of the turbine, liquid hydrazine is pulsed into the catalyst bed rather than supplied in a continuous flow. The speed of the turbine is controlled by varying the rate at which the liquid hydrazine is pulsed into the gas generator, increasing the pulse rate to increase the speed of the turbine, and decreasing the pulse rate to decrease the speed of the turbine. It will be recognized that since the system may operate a generator required to operate at a particular frequency, having precise control of the speed of the turbine, and the volume and other characteristics of the gas produced by the gas generator, is an important requirement.
While the technique of pulsing the liquid hydrazine into the gas generator is highly desireable due to the precise degree of control possible over the power output of the device, the pulsing operation has proven to be extremly destructive to the catalyst beds, drastically limiting the life of the gas generator. The hydrazine liquid pulsed into the gas generator causes an oscillatory movement in the catalyst beds resulting in destructive vibration of the catalyst, which is in the form of particles having a particular size and shape. The vibration in the catalyst beds causes the particles to be broken down into less useful, smaller particles, and finally into dust. The vibration problem is particularly destructive in the primary bed, which is situated immediately adjacent the spray nozzles through which the liquid hydrazine is introduced into the gas generator. As the catalyst particles in the primary bed break up, the dust produced migrates into the secondary bed, which becomes clogged rapidly by the dust, thus increasing the pressure drop across the gas generator. The vibration problem in the primary bed is so severe as to cause significant abrasion on the interior of the gas generator.
In order to minimize this problem, in the past biasing springs have been used to force the secondary catalyst bed, typically enclosed in a porous cylindrical structure, against the primary catalyst bed to compensate for compaction, bed degradation in the primary bed, and/or manufacturing tolerances, and to reduce the amount of vibration occurring therein due to the pulsing hydrazine spray after an initial use of the system. While this technique allows reuse of the gas generator without the necessity to completely rebuild the generator after each use, the number of uses the generator is capable of without requiring rebuilding and replacement of the catalyst is still far less than satisfactory. The initial degradation of the primary catalyst bed is somewhat slower when springs are used to bias the secondary bed against the primary bed, but after initial wear in the primary catalyst bed occurs the catalyst degrades at a quickly increasing rate.
Another factor in the problem is the ambient temperature at which the gas generator is operating. While catalyst degradation is merely unsatisfactory at ambient temperatures of 70.degree. F., at lower operating temperatures the catalyst breakdown rate drastically increases to make the expected lifetime of the gas generator particularly short, rendering the device no longer just unsatisfactory but rather unacceptable. While the number of multiple starts possible at a low ambient operating temperature is a maximum of 5-10, it must be noted that the performance of the gas generator as an engine start device sharply diminishes resulting in a markedly slower turbine acceleration, increasing start time, and drastic catalyst weight loss through increasing oscillatory vibration.
Since one of the most important applications of the gas generator is as an emergency power source for a military aircraft, which typically operates at a high altitude having a low ambient temperature, it can be seen that this type of gas generator will have a fairly short operating life after which a complete rebuilding and replacement of the catalyst bed is necessary. Since the gas generator has application as a component in the emergency power system for the aircraft, any failure in the gas generator could result in loss of the aircraft due to failure to restart the engine or operate the electric or hydraulic system of the plane. It can therefore be appreciated that the gas generator would be required to be rebuilt after virtually every use to ensure that loss of the aircraft does not result from decreased performance of the gas generator due to catalyst degradation from oscillatory vibration.
Although failure of the gas generator resulting in possible loss of an aircraft is of paramount importance, another major concern in premature catalyst degradation is the high cost of rebuilding the gas generator and replacing the catalyst. To rebuild the gas generator, it is necessary to remove the generator from the aircraft and completely disassemble it to replace the catalyst. Such an operation is labor intensive, requiring a substantial amount of time from a skilled technician. In addition, the Shell 405 or like catalyst used in the gas generator is extremely expensive. Although only a small amount of catalyst is contained in the primary and secondary beds of the gas generator, the cost of even the small amount of catalyst may be greater than the cost of the mechanically complete generator. Finally, an additional cost in military applications is that the aircraft must either have a large number of spare gas generators on hand, or else be unavailable for service while the gas generator is being rebuilt.
Thus, it can be seen that a strong need exists for a gas generator having an extended catalyst life allowing the generator to be reused a large number of times while requiring only minimal maintenance between uses. The gas generator must protect the catalyst particles from destructive oscillatory vibration to ensure against loss of the catalyst in the primary bed and clogging of the catalyst in the secondary bed. The gas generator must be capable of functioning reliably at low temperatures, and of providing multiple starts at these low temperatures. Finally, it is desirable that whatever solution is found be adaptable to retrofit existing gas generators to overcome the above-described problems.